The large off-the-road pneumatic tires used in heavy construction and earthmoving operations have operating loads and inflation pressures much higher than conventional trucks and lightweight vehicles. Therefore, the radial plies earthmover tires need particular reinforcing cords.
Since the steel cord for reinforcing the tire is required to have high breaking load and high fatigue resistance, conventionally a steel cord having a multi-strand structure is widely used. A multi-strand structure has multiple strands each composed of a plurality of steel filaments that are twisted together. The strands are twisted together to form the cord. For example, a steel cord having a 7×(3+9+15)+1 structure, as shown in FIG. 1 is known. However, the structure 7×(3+9+15)+1 has a relatively low fatigue resistance.
Besides, the other concern for the performance of the off-the-road tire is insuring adequate rubber penetration into the cords. During the manufacture of the belt layers and in the subsequent tire vulcanization rubber is expected to penetrate into all voids between the filaments so as to assure an adequate corrosion protection. Unfortunately, the conventional structure 7×(3+9+15)+1 shows in general insufficient rubber penetration.
EP 0 602 733 B1 discloses a multi-strand steel cord comprising a core strand and up to nine peripheral strands surrounding the core, each strand having a centre of one or more centre filaments and two or more layers of filaments surrounding the centre. The steel cord of this patent has an adequate rubber penetration which is obtained by providing free spaces between the individual filaments after careful choice of the twisting angles.
The twist angle of a layer is within the context of EP 0 602 733 B1 defined as follows. Suppose that d1 is the (total) diameter of the center, that d2 is the diameter of the filaments of the radially inner layer which immediately surrounds the center and that d3 is the diameter of the filaments of a second layer surrounding the radially inner layer (=radially outer layer).
LL2 is the lay length of the radially inner layer and LL3 is the lay length of the radially outer layer.
The twist angle of the radially inner layer is defined as:α2=arctg [(d1+d2)×π/LL2]×180/π
The twist angle of the second layer is defined as:α3=arctg └(d1+2×d2+d3)×π/LL3┘×180/π
JP2006-104636A discloses a steel cord reinforcing rubber products, which comprises one core strand of layer twisting structure of two or three layers and six sheath strands of layer twisting structure of two or three layers surrounding this core strand. All the layer twist directions in core strand and the twist direction of sheath strands are the same, but the layer twist direction in sheath strand consisting of combination of different directions, which results in large loss of tensile strength and large loss of breaking load.